1. Field of the Invention
The present invention relates to a structure of a defogger formed on a window glass panel of a motor vehicle, particularly to a heating line pattern structure of a defogger formed on a window glass panel on which an antenna is provided
2. Related Art
When a TV (television) antenna, particularly a digital TV antenna for 470˜710 MHz band is provided on a rear window glass panel on which a defogger comprising a plurality of heating lines arranged between two bus-bars is formed, a problem is caused such that the control for the directivity of an antenna to a target electric wave becomes difficult due to an effect by the heating lines of the defogger. This is due to the fact that the directivity of an antenna on the rear window glass panel is varied in a vertical direction as shown in FIG. 1 by the dotted line 8 to decrease the sensitivity of the antenna in a horizontal direction.
In order to resolve such a problem, the applicant of the present application has proposed that a part of heating line of a defogger opposed to an antenna is patterned as a meander shape in Japanese Patent Publication No. 2002-8081.
FIG. 2 shows a heating line pattern having a meander shape proposed in the above-described publication. In the figure, the heating line pattern is symmetrical in right and left, so that the left end portion of the pattern is only shown for the simplicity of the drawing.
A monopole antenna 16 for a digital TV is provided between a defogger 12 on a rear window glass panel 10 of a motor vehicle and a roof 14 of a vehicle body, and on a side portion neighbored to a pillar 17. Reference numeral 18 designates a feeding point of the monopole antenna 16. The defogger 12 and monopole antenna 16 are made with silver print lines that are formed by printing a silver paste and then firing the paste.
The defogger 12 is structured by bus-bars 20 on both side portion and a plurality of heating lines arranged between the bus-bars 20. A part of the upper most heating line 12-1 neighbored to the monopole antenna 16 has a meander shape. The heating line is meandered in a rectangular manner at regular intervals to form a meander shape heating line portion 22.
Under the meander shape heating line portion 22, one lateral heating line 13 is extended and connected to a vertical heating line 15. Four lateral heating lines 12-2, 12-3, 12-4 and 12-5 are commonly connected to the vertical heating lines 15. The lateral heating line 13 under the meander shape heating line portion 22 is on the same horizontal position as the heating line 12-5. A plurality of heating lines under the heating line 12-5 are normal linear heating lines 12-6 extending between the bus-bars 20.
In order to increase the defogging effect of the heating lines of the defogger 12, a meander shape structure is generally adopted for the heating lines. For example, Japanese Utility Model Publication No. 1990-139161 disclosed that a heating line is formed in a zigzag manner, the pitch of zigzagged line being made small in a region required to be strongly heated.
However, the meander shape structure shown in FIG. 2 has been adopted to decrease the effect of the heating lines to an antenna provided neighbored to a defogger, so that the meander shape structure proposed by the present applicant is different in concept from that disclosed in the Utility Model Publication.
In the heating line pattern of the defogger shown in FIG. 2, the currents through the four heating lines 12-2, 12-3, 12-4 and 12-5 flows into the heating lines 13 and 15 under the meander shape heating line portion 22, so that the width of respective heating lines 13 and 15 is selected to be three-four times that of a normal heating line to suppress the increasing of heating due to the lines 13 and 15. In this case, the width of respective heating lines 13 and 15 is too thick, so that the heating lines have a poor-looking.